Thanks Junchao.
I don't see you measure MPI_Allreduce.
You're right, let's call my code "a simple example to reproduce a bug" rather than a benchmark.
Basically you only measured some random > numbers across processes.
The usleep simulate the time to perform computation in my real code (Computational Fluid Dynamic software). bench_mpi.cxx only do a usleep(microseconds) then call MPI_Allreduce. microseconds is a constant base time, divised by mpi_size (+ a random overhead between 0% and 5%, so that MPI_Allreduce is not called at the same wall clock on all proceses, but I thing a should have use a different seed on each proc). So because what the code do is only usleep(base_time / mpi_size), I expect the wall clock time to be half with twice processor. With MPiCH 3.1.4, the wall clock time increase with 7 or more processes. MPI_Allreduce become very slow without a reason. I'm triying to understand why. _______________________________________________ discuss mailing list [email protected] To manage subscription options or unsubscribe: https://lists.mpich.org/mailman/listinfo/discuss